Although a variety of parasites which prey upon these larvae is not large, they are very numerous in individuals, and it is to this cause that only a small per cent of the larvae ever produce moths. These parasites develop beneath the skin of the larva as footless grubs, which, at first, do not attack the vital organs, but later these organs are preyed upon, and the larva dies. A wasp-like insect which preys upon this larva well illustrates in its habits the crudeness of many instincts. The female will lay eight or ten eggs upon one caterpillar, but as the young parasitic grubs require a large amount of food, only one is able to mature and the others perish.
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| PROMETHEAN MOTH. (Callosamia promethea) | ||
| Adult Male | Adult Female | |
| Larva. | Pupa. | Cocoon |
| CECROPIAN MOTH. (Samia cecropia.) Pupa. | ||
| Adult Male. | Eggs on Maple Leaf. About ½ Life-size. | Adult Female. |
| Larva. | Cocoon | |
| COPYRIGHT 1900 BY A. W. MUMFORD, CHICAGO. | ||
The insect parasites seem, in many cases, to mature and transform into the adult stage after the caterpillar has built its cocoon, and thus many parasites lose their lives, since they are not always able to escape from the cocoon. A cocoon will sometimes be found filled with these small insects, which have not been able to make their escape, and have thus died in prison.
The adult larva, unlike Prometha, usually spins its cocoon not attached to a leaf, but along a stem; sometimes, however, they may be placed in other situations.
In about two weeks after the cocoon has been spun, the larva transforms into a chrysalis, in which stage it hibernates during the winter, and from which it emerges in May or June.
During the winter, when the leaves are not on the trees and shrubs which are frequented by these larvae, a large number of cocoons may easily be collected. These should be kept out of doors during the winter, for if kept in a warm room they will emerge during the winter or so early in the spring that food cannot be secured for the larvae.
If one secure a number of old cocoons, from which the moths have failed to emerge, and cut them open longitudinally, he may learn many interesting facts. A dead and dry mummified looking larva or chrysalis may be found, or, what is even more interesting, no trace of the larva or chrysalis may be present, but only a mass of small white, paper-like cocoons. These have been left by a colony of little wasp-like parasites which may occur in such large numbers that there is scarcely room for all to spin their cocoons, so that on account of being so closely crowded together, they are moulded into a mass of cocoons having the form of the cavity formerly occupied by the larva.
The cocoons of Cecropia are composed of two parchment-like layers of silk which are generally very dense and strong. The space between these two layers contains loosely spun threads of silk like a layer of packing material. The larvae seem normally to make three varieties of cocoons; one kind is very loosely constructed, much larger than the ordinary form and not attached to a twig, but found in the grass or in shrubs near the ground. The two other forms of cocoons are much smaller and more closely woven, but differ in size; female moths as a rule emerging from the larger cocoons, and males from the smaller ones.
Dead larvae are sometimes found in cocoons which are practically of a single thickness; there being no space between the outer and inner layers. The hollow skins of the larvae found in such cocoons clearly show that this unusual cocoon is due to the influence of parasites upon the larva.
In the upper open end of the cocoon, kernels of wheat, corn, beechnuts and even acorns have been found. How these get in this position seems to be quite a puzzle. In opening twenty or thirty cocoons, five or six kernels of corn have been found, thus showing that this occurrence is by no means rare. Chickadees and blue-jays have been given the blame for this work, since these birds are thought to have the habit of hiding food. The inverted outer layer of the cocoon clearly shows, in some cases, that the kernel of corn has been thrust into the cocoon with some force.